1. Field of the Invention
The present invention relates to a power zoom lens, and a camera system having such a power zoom lens.
This application is related to the commonly assigned applications U.S. Ser. No. 410,880, No. 652,038, No. 407,424, No. 414,477, No. 414,478, No. 670,733, No. 406,523, and No. 414,476, the disclosure of which are expressly incorporated by reference herein.
2. Description of Related Art
A power zoom lens having an automatic focusing device and a power zoom device, and a camera system having such a power zoom lens are known. Also, special photographic technologies in which the power zoom lens is employed have been developed. For instance, as disclosed in U.S. Ser. No. 07/410,880 the assignee of which is same as that of the present application, a constant image magnification zooming means in which a set image magnification is kept constant is known. In this method, after the image magnification is set, a target focal length, at which the image magnification is constant, is obtained by calculation in accordance with the amount of defocus of an object to be taken. The power zooming is then carried out in accordance with the target focal length thus obtained.
However, in a single lens reflex camera, since a coefficient (which will be referred to as a "K value" hereinafter) for an angular displacement of the AF motor necessary to move the focal point by a unit displacement varies in accordance with the focal length, the defocus amount varies during the zooming operation. Consequently, it is necessary to correct the defocus amount, in view of the change in K value, to obtain a correct target focal length. However, this results in a complicated calculation.
The error of the defocus amount increases as the focusing lens unit moves away from the focal position. In view of the increase in error of the K value, a backlash of the lens driving system, and an error of measurement of the angular displacement of the motor, etc., it can not always be guaranteed that the focusing lens unit will be moved to the focal position by one calculation. As a result, there is a possibility of an occurrence of a searching operation in which when the focusing lens unit stops before the focal position, a further movement thereof in the same direction is necessary, and when the focusing lens unit moves beyond the focal position, a returning movement thereof in the opposite direction is necessary. Under such circumstances, every time the focusing lens unit stops at a position other than the focal position, the target focal length must be recalculated.
As can be understood from the above discussion, neither a quick calculation nor a smooth operation of the focusing lens unit can be effected by the known method in which the target focal length is calculated based on the defocus amount, since the calculation values necessary for the focusing control and those necessary for the zooming control interfere with each other.